In recent years, a research for a wireless sensor network through which data collected for various applications are wirelessly transmitted to a high-order network has been performed by using a wireless device in which a small-sized wireless module is mounted on a sensor. It is assumed that the wireless sensor network is used in various scenes such as a medical front, a farm, or a town. As an example of a wireless device, there is a sensor node in which a sensor collects information such as an object, a person, or an environment and a wireless module wirelessly transmits data.
Now, it will be explained about an application example of a wireless sensor network with reference to FIG. 15. FIG. 15 is a diagram explaining an application example of a wireless sensor network. In FIG. 15, it will be explained about the case where a wireless sensor network is applied to a medical front.
For example, as illustrated in FIG. 15, the wireless sensor network includes a plurality of sensor nodes that includes a wireless module and a sensor that are integrated and a coordinator that aggregates sensing data transmitted from the sensor node. As an example of a wireless communication technology using a sensor node, there are PAN (Personal Area Network), BAN (Body Area Network), and the like (hereinafter, “PAN/BAN”).
In the PAN/BAN, there are, for example, an environmental sensor node that senses a temperature, a humidity, or an illuminance of surrounding environment, a vital sensor node that senses a temperature, a blood pressure, or a pulsebeat of a person, and the like. Moreover, a sensor node includes a node that is located on a human body and a node that is located within a range of a communication area that is about 10 meters around the human body, in the medical front.
On the other hand, a coordinator transmits data aggregated from each sensor node to a plurality of sensor nodes or AP (Access Point) that is located in a communication area. Moreover, the AP that receives data from the coordinator transmits the received data to a high-order network. At this time, the AP performs beam forming for transferring a radio wave to a specific communication area such as each sensor node or a coordinator in a focused way and performs wireless communication with each sensor node or a coordinator.
However, power delivery to the sensor node can use a built-in battery such as a button battery in some case assuming that the sensor node is used in the outdoors. Therefore, when a sensor node is used in the outdoors, it is particularly desirable that power consumption can be suppressed in terms of the lifetime of a battery. For this reason, for example, a sensor node for a wireless sensor network employs a protocol for making the node enter a sleep mode when the node is not used in many cases in order to last a battery for a long time by the suppression of power consumption.
On the other hand, the AP includes, for example, a plurality of array antenna, and controls an amplitude, a phase, or the like that acts as an array weight of each array antenna at the time of transmission and reception of a signal to turn the direction of beam of antenna to an arbitrary sensor node that is a communication target. Moreover, the AP performs null steering for turning low-level directivity to an interference source in the communication with an arbitrary sensor node. In this way, the AP raises the transmission quality of a signal that is transmitted and received.
When each sensor node transmits a signal, the wireless sensor network system can suppress power consumption of each sensor node in accordance with a gain improvement effect of a wireless communication path of the AP and each sensor node when beam directivity of AP is performed. The technology has been known as disclosed in, for example, Japanese Laid-open Patent Publication No. 2006-191514.
However, there is a problem in that the reliability in communication cannot be secured in the above conventional art.
As a specific example, it is assumed that a transmission range is about ten meters in short-distance wireless communication such as PAN/BAN. Moreover, the connection to a different external network such as a wireless LAN (Local Area Network) is performed within ten meters in a communication method used in PAN/BAN.
As described above, AP that connects a wireless device and an external network improves communication quality with the wireless device that is a communication target in accordance with the improvement of gain by the control of a communication path using the beam directivity of an array antenna. However, when an obstacle that blocks a communication path of a radio wave enters between the wireless device that is a communication target and the AP, the wraparound of a radio wave becomes weak by narrowing a beam and a communication cutoff can be performed by degrading a communication state.
As illustrated in FIG. 16, to avoid a communication cutoff caused by a factor such as an obstacle that blocks a communication path, it is conceivable that direct communication between a wireless device X and AP is changed to communication performed by a reflected wave, by a wall, of a signal transmitted from the wireless device X, for example. At this time, the AP has beam directivity to a reflected wave formed by a wall. FIG. 16 is a diagram illustrating an example of avoiding a communication cutoff caused by a factor such as an obstacle that blocks a communication path.
However, communication by a reflected wave formed by a wall and the like is unstable compared to direct communication between a wireless device and AP and is communication having low reliability in which a signal is degraded to some extent. Moreover, in outdoor wireless communication, it is difficult that a suitable reflected wave is always obtained from a wireless device that is not fixed and another communication path for avoiding a communication cutoff cannot be secured in some cases.
Furthermore, when using a wireless device in a medical front or the like, it is preferable that an instant communication cutoff for path switching is also avoided. As a result, the above conventional art cannot secure communication reliability.